Apparatus and method of solder coating integrated circuit leads

ABSTRACT

Desired control of the thickness and composition of a solder coat on the J-leads of an integrated circuit Quad package is obtained by orienting the packages while being solder coated in a &#34;leads-up&#34; orientation as a series of strips mounting the packages are passed through a solder wave of a wave soldering apparatus on a pallet. The Quad or other shaped integrated circuit or other electronic packages which have leads extending exteriorly thereof, thus have the critical lead crest portions coated with substantially the same solder layer thickness and composition. This permits reliable electrical connections between the crest portions of the leads and printed circuit board metal traces (metallization), particularly in surface mounting of the package to a printed circuit board. An additional feature of the invention is a pallet for holding a series of package-holding elongate frames which pallet is used for conveying the packages through the wave soldering apparatus.

This application is a division of application Ser. No. 794,038 filedOct. 31, 1985, now U.S. Pat. No. 4,657,172 issued Apr. 14, 1987.

FIELD OF THE INVENTION

This invention relates to the method of placement of a solder coating onmultiple metallic leads extending from an integrated circuit package.More particularly, it is directed to a procedure and multichip palletfor ensuring an essentially constant thickness of a resultant solderlayer on critical areas of each of the leads and where the solder layeris essentially homogeneous in composition over the entire solder layeron the multiple leads.

BACKGROUND OF THE INVENTION

In general, in both DIP (dual-in-line package) and plastic leaded chipcarriers (PLCC or Quad), solder plating using electrical depositiontechniques have been employed to plate a thin layer of solder, typically60% tin and 40% lead. However, it has been difficult to obtain uniformdeposits over the entire surface of the multiple leads extending fromthe plastic package. This is particularly so with respect to so-calledJ-leads, which extend from a midpoint of the four sides of a Quadpackage and are bent downwardly around the package bottom edge andextend inward to form the bottom of the J. The curved bottom hereinafteris called the lead "crest". Further in solder plating, it is verydifficult to control the solder composition being plated over the entiresurface of the multiple leads resulting in nominal 60-40 (Sn-Pb) solderhaving areas of 85% Sn-15% Pb or 80% Sn-20% Pb for example.

A nonuniform thickness of the solder layer at a J-lead crest results inan unreliable joint between that lead when it is to be solder mounteddirectly on copper traces or conductive pads of a printed circuit board,or the like, in so-called surface-mounted technology. This occurs whenan "over thick" layer at one lead crest creates a high point on thedesired plane of lead crests forming the overall package, for example."Under thick" crests, or indeed layers of correct thickness, thus, donot make good contact with its trace. It is critical that the bottomsurface of the crests of all leads, for example the 84 leads in atypical Quad package, are substantially all in the same plane. Inaddition, excess solder can cause bridging upon board mounting.

Due to the above nonuniform thickness and nonuniform compositionproblems in solder plating and the difficulties of controlling thicknessand composition, one of the world's largest user of these devices (IBM)has specified that a solder dip process be utilized for coating solderon integrated circuit package leads since it was believed that a better,more reliable solder joint between the lead and a solder pad on theprinted circuit board would result. Effort has been made to utilize aso-called wave soldering machine in placing a solder coating on J orother type leads by a dip process. In such a machine, a holder ofintegrated circuit packages, all in a single file end-to-end, isconveyed through a fluxer station, a preheater station and the wavesolder station. The wave solder station contains a solder pot, a pumpand a nozzle which forms the actual wave of solder through which theholder is passed. Wave soldering machines may be horizontal or inclined,the latter where the conveyor rides up an incline from the flux stationto the preheater station to the wave soldering station.

As is known in the field, oil intermix (U.S. Pat. No. 3,098,441) may beused in wave soldering to inhibit dross (oxides of Sn and Pb) formation.It is known that the intermixed oil aids in the post-soldering removalof corrosion causing flux residues. The conventional solder coat processin the past has been to dip or pass the overall package through a solderwave in a leads down orientation.

End users mount and reflow the packages with the resultant dipped soldercoated leads onto a board by conveying the chip carriers though a vaporblanket utilizing vaporized FC-70 Fluorinert liquid (3M Co.). Since themelting point of 60Sn-40Pb solder is 185° C., a vapor phase zone ofFC-70 at 216° C. is appropriate.

SUMMARY OF THE INVENTION

With respect to soldering Quad 84-leads integrated circuit packagesutilizing an ASTRA 16 wave soldering system of Hollis Automation, Inc.,it was not possible to meet IBM solder thickness and solder compositionspecifications by passing the package "pins down" across the solderwave. Resultant tests on leads coated in the "down configuration"indicated many lead crest thicknesses of over 1000 microinches when amean of 500 microinches was desired. Further evidence indicated thatsolder icicles or balls extending from the crests and bridging of solderbetween leads occurred in such processing. Additional testing indicateda wide variance of solder composition on various leads ostensiblysubjected to the same solder dip for similar times and temperature.Variations of from 56% Sn to 100% Sn were observed at various leadcrests on the same integrated circuit package. It was theorized that asthe package left the wave or in the subsequent reflow step, the moltensolder flowed from the dip package surfaces by gravity down the leadtoward its J or other tips building up a greater variation of solderlayer thickness than is desired. Variations in solder composition alsoresulted.

It has been found that when the circuit packages are conveyed throughthe wave step and any reflow step while the DIP or J-type leads faceupwardly, an "up configuration" with respect to the base of the wave,specifications on crest thickness and composition were met. Furtheruniformity of the solder surface, including no dewetting of solder onthe leads, with freedom from nodules, blisters, flakes and cracksresulted. Uniformity of solder thickness on the crest area of each leadallowed coplanarity requirements of the customer to be met.

It is theorized that, as the packages leave the solder wave and arereflowed in a subsequent vapor phase zone, the molten solder flowsdownwardly out under the leads past the plane of the plastic packaging,i.e. the solder flows toward the package and away from the upwardlyfacing crests rather than flowing off the crest tips away from thepackage. It is believed that sufficient surface tension is present inthe solder to hold the desired thickness of solder on the outer crestsurfaces. These surfaces are the point of attachment of the lead to theconductive trace on the printed circuit board as applied insurface-mount technology. It is not overly critical that extra thicknessof solder coat is on the noncrest portions of the lead as long as thereis no bridging of solder between adjacent leads.

The critical area of a lead is the outer surface of the lead crest whichis to be surface mounted connected to a PCB trace. In making the PCBconnection, the solder on the J or other type lead, when properlyaligned and fluxed, is melted, such that it flows down around theconnection forming a miniscus of solder around the newly formed joint.Since outer surfaces of the crests of each lead, processed in the"leads-up" orientation, are all appreciably in the same plane (no highlydisparate solder thicknesses) that plane is parallel to the PCB tracesor pads on the PCB to which they are to be attached. As a result, eachof the simultaneously-made 84 joints in an 84-lead carrier are allwithin specifications.

The invention may be carried out by preferably mounting a series,typically five, of printed circuit carrier packages in an elongate framecrosswise in a novel pallet which then holds and conveys a series ofelongate parallel frames transversely of the solder wave. By suchtransverse orientation, each package in each elongate frame is contactedat the same time and at the same temperature across the solder wave. Ifthe packages are serially advanced in single file across the wave, eachpackage tends to cool the wave and not be dipped simultaneously, thus,affecting the resulting thickness and composition of the solder coatingon the leads.

Lastly, the invention includes optimizing the parameters of a wavesoldering machine to aid in achieving the results sought, namely,preventing a wide variation in solder thickness and solder compositionparticularly at the critical connect-to-trace area of an integratedcircuit package lead.

The present invention not only meets a major customer specificationrequirement, but also reduces manufacturing costs through better yieldsand meets industry requirements for surface-mounted printed circuitboard assembly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view of a prior art wave soldering apparatus.

FIG. 2 is a schematic top view of the apparatus of FIG. 1.

FIG. 3 is a perspective view of a prior art J-leaded integrated circuitpackage.

FIG. 4 is a side view of the package of FIG. 3 with one lead andsurrounding casing in partial cutaway cross-section.

FIG. 5 is a schematic side view of adjacent J-leads and a printedcircuit board surface-mount with the adjacent leads rotated 90° forillustrative purposes.

FIG. 6 is a side elevational view of an elongated frame or stripmounting a series of five integrated circuit packages of the J-lead Quadtype.

FIG. 7 is a top view of the elongated frame and packages of FIG. 6.

FIG. 8 is a perspective view of a production pallet for mountingmultiple parallel series of five package elongate frames.

FIG. 9 is a partial side view of the pallet taken on line 9--9 of FIG. 8showing the packages supported for solder coating in a "leads-up"orientation.

FIG. 10a is a top view of a pallet package-support bar.

FIG. 10b is a side view of the pallet package-support bar showing itssupport of a frame-mounted package.

DETAILED DESCRIPTION

FIGS. 1 and 2 schematically illustrate a wave soldering apparatuspreferably used in practising the inventions set forth herein.Basically, the apparatus 10 comprises an inclined conveyor 11 for movinga pallet 12 of elongate frames of electronic packages or otherelectronic parts to receive a coating of solder. The package or parts,first pass above a first station comprising a flux container 14 whichincludes top baffles and a pump (not shown) to form a fluxing zone 15above the level of flux in the container. The package leads or parts arethus coated with a flux. The packages or parts continue up the inclineof the conveyor through a second station comprising a radiant preheater16 forming a heating zone 20 to preheat the packages, or parts, andleads to avoid subsequent thermal shock from the application of moltensolder. Solder coating occurs at a third station comprising a moltensolder container 17 having a solder wave-producing series of baffles andpump 18, as is known in the art, to form a wave 19 of molten solder withintermixed oil above the solder level in the container 17. The packagesand leads or other parts pass through the wave. A speed controller on apanel board controls the conveyor speed. The wave is contained bybaffles 21 which give the wave a rectangular shape.

In practice, packages or parts are loaded onto a pallet or other fixtureholding the parts to the conveyor for transport through theabove-described apparatus. An activated water soluble flux (Blackstone1452) is applied to the package leads in zone 15 using foam fluxer (orwave fluxer) and any excess flux removed by an air knife (not shown).The parts are heated in the preheater zone to evaporate water moisturefrom the flux solution, afford the flux an opportunity to remove oxidesfrom the surface of the electrical leads and to raise the temperature ofthe package and leads to minimize thermal shock when they pass throughthe solder wave. The wave is generated by pumping liquid solder(nominally 60% Sn and 40% Pb) up between baffles 21 to form a wave zone19. Heretofore, the packages or parts were passed in a leads-downorientation in a serial-in-line progression along the incline of theconveyor through the wave so that basically only the depending leadswere below the wave top. Immersion time has generally been from twoseconds to three seconds. The fixture holding the series of integratedcircuit packages may be dipped in the "leads-down" orientation in theindustry accepted prior art wave soldering process to reflow thedeposited solder on the leads. This reflow technique is detailed inAssembly Engineering, June 1977, in an article by T. Thompson. A ModelIL-6 apparatus sold by HTC Inc., Concord, MA, may be employed forperforming the reflow step.

FIG. 3 illustrates a typical Quad-type integrated circuit package 22having a flat surface 23 surrounded by castellated peripheral edgeportion 23a between which J-leads 25 are bent and form the J-lead crests25c. The overall package 22 is subsequently surface-mounted to printedcircuit board traces at the crests 25c. Other types of electronicpackages such as the "pocket" type where the free end of each lead isbent into a rectangular or other shaped depression or pocket in andaround the periphery of package surface 23 may be processed with thisinvention.

FIG. 4 illustrates the difficulties which were encountered in the priorart solder coating of the package of FIG. 3 when the solder coating wasperformed in a Hollis wave soldering machine (ASTRA 16) with the J-leadsfacing downwardly during and immediately after the solder dip operation.The proximal inner ends 25a of the leads 25 are attached tometallization on a chip lead frame 27 which is wire bonded to thecontact pads of an integrated circuit chip 28 inside the package 22.Package 22 is molded or encapsulated with an epoxy resin with leadportion 25a extending from the side edges of the package at theperiphery 26c of the package. A downward lead portion 25b is essentiallyvertical with respect to the horizontal package. The bottom curvedportion of the J-lead forms a bottom crest portion 25c, the outersurface of which is eventually to be surface-mounted to a PCB trace anda distal end 25d which abuts or is in a pocket in the package surface 23between castellations 23a. The crests 25c extend between and exterior ofthe top of the castellations 23a. Upon "leads down" solder coating, theJ-lead crests are coated with a relative wide variance of solderthicknesses. For example, some leads are coated thickly as at 25e, whileother leads are much thinner as at 25f. Since it is desired that thecoated crests have the exterior surfaces all in the same plane, i.e.planar, it is apparent that in the illustrated FIG. 4 package the soldercoating would be out-of-specification. As will be shown, the coatedcrests formed in a "leads down configuration" also have been found tovary extensively in solder composition.

FIG. 5 illustrates the affect of overly thick crest coatings or oficicles or solder balls on the lead crests for the end user. A pair ofadjacent J-leads 25 (rotated 90° from their usual orientation with theedge of a package 22) are shown with their crests 25c in a planeparallel to a metal path or traces 32a and 32b on a surface 31 of aprinted circuit board 30 to which the leads (and package) are to besurface-mounted. A coating of flux 33 is present over the traces to aidin forming a proper solder connection between lead crest 25c and traces32a and 32b when the leads are placed in position over the traces forconnection. Overthick solder coated crest 25e on the right-hand crest25c makes first contact with the trace 32b and prevents the adjacentleft hand lead crest with its thinner solder coat (and other adjacentlead crests) from making proper contact with the trace 32a. The distanced₂ between the bottom of the solder coat on the left lead and thesurface 31 is thus greater than the distance d₁ between the bottom ofthe solder coat on the right lead and the surface 31 and a gap 34 isformed between the left-hand crest coated surface and trace 32a. Whenthe leads 25, solder coat 36, adjacent flux 33 and traces 32a and 32bare heated to soldering temperature, the solder 36 on the leads flowsdownwardly to form a solder miniscus 37 shown by the dotted outline. Dueto the noncontacting of the left-hand lead 25, more particularly itscoated crest 25c, with trace 32a, an imperfect joint is madetherebetween with a high possibility of air and bubble inclusions.

FIGS. 6 and 7 illustrate the desired orientation of multiple plasticchip carrier and integrated circuit packages 22 when the package leadsare to be solder coated by passing a strip of packages through asoldering wave. Each package is attached to an elongated frame or strip40 having lateral edges 41 with indexing perforations 42 therein forautomatic machine handling. As seen in FIG. 8, a pallet mounts thestrips and includes a series of short pins 55 extending upwardly fromsurfaces 56 onto which a strip of packages having side apertures 46 isplaced so that the apertures 46 are aligned with and inserted over pins55 on adjacent bars 53. Each package is held at its corners byconnection links or tie-bars 45 extending between the edges 41 and anintegral portion of the lead frame strip within the package. After wavesoldering, the links or tie-bars are severed so that the five packagescan be removed from the frame. Cutouts or openings 41a and 41b areprovided in the frame.

A feature of this invention is that a group of five or more packages 22,are advanced at one time across a solder wave at a right angle to strip40 as shown by arrow 44 in FIG. 7. Each of the leads 25 of packages 22are in a "crests up" orientation. Such orientation when the packages arepassed through the wave soldering apparatus (FIGS. 1 and 2) results ineven (within specification) coating of the lead crests with a resultantnarrow range of solder composition at the lead crest critical area. Eachof five packages contact the solder wave simultaneously and creates aminimum change in temperature of the solder across the wave. If thestrip 40 is put into the wave lengthwise, each individual package lowersthe wave temperature and gives a different coating thickness ondifferent packages.

FIG. 8 illustrates a production pallet which is the preferred best modeof placing and holding the strips 40 for conveyance through the wavesoldering machine. The pallet 50 is preferably made from titanium forits lightness, strength and corrosion-resistant properties. Pallet 50comprises two lengthwise elongate support members or bars 51a and 51bhaving a top surface 52 and a series of spaced support crossbars 53between bars 51a and 51b. Each crossbar includes pins 55 as discussedabove for holding the apertured adjacent edges of the elongate frames towhich the packages (FIGS. 6 and 7) are attached in a "leads up"orientation. The pallet rigidly clamps the frames, allows forsimultaneous plating of multiple packages and allows a large number offrames to be supported in a single structure.

As seen in FIG. 8, package support bars 60 (one shown without a stripunderneath) extend parallel to and between each of two adjacentcrossbars of the pallet and are connected to the bars 51a and 51b of thepallet by suitable fasteners 63 extending through apertures 64 and 64a(FIG. 10a) in the lengthwise bars and support bars, respectively.

Each support bar 60 (FIGS. 9 and 10b) has a series of sets of at leasttwo downwardly extending pins which, when the support bars are assembledover the strips 40, abut each of the topsides of the packages 22 inwardof the leads 25 at at least two spaced points 65 and 66 to support eachpackage in the row of the packages 22. The packages and frames aresuspended transversely of bars 51a and 51b and extend parallel betweencrossbars 53. Support bars 60 support each frame and each package sothat the frame and package does not bulge upwardly and flex when theyand the pallet are passed through the solder wave.

In solder coating operations using the preferred ASTRA 16 machine, theoperator first assures that the preheater temperatures are at thedesired level. After conveyor start-up, the temperature of the upwardlyfacing surface of the package is checked. Care must be taken to have thespecific gravity of the flux in the flux container kept within themanufacturer's specifications. The following operating parameters of theASTRA 16 machine have been found to be optimum:

    ______________________________________                                                                 OPER-                                                               STAN-     ATING                                                PARAMETER      DARD      RANGE      CHECK                                     ______________________________________                                         1. Flux Specific Gravity                                                                        1.129     ±0.010 every                                                               4 hrs.                                            2. Flux Air Pressure -                                                                          1.0       ±0.1 PSI                                          Foam                                                                       3. Flux Air Pressure -                                                                          1.5       ±0.1 PSI                                          Spray                                                                      4. Air Knife Pressure                                                                           2.0       ±0.2 PSI                                       5. Temperature @ Pre-                                                                           120° C.                                                                          ±5° C.                                     heater                                                                     6. Package Body Tem-                                                                            180° C.                                                                          ±5° C.                                     perature @ solder                                                             wave                                                                       7. Solder Temperature                                                                           255° C.                                                                          ±5° C.                                  8. Solder Wave Height                                                                           0.4 inch  ±0.5                                           9. Solder Wave Width                                                                            1.7 inch  ±0.3                                          10. Conveyor Speed 2.5 Ft/Min                                                                              ±0.5 Ft/Min.                                  11. Conveyor Angle 6.3°                                                                             ±0.3                                          12. Input D.I. Water                                                                             2.5 GPM   ±1/2 GPM                                          Flow rate                                                                 13. Rinse Water Temper-                                                                          58° C.                                                                           ±2° C.                                     ature                                                                     14. Water Pump Pressure                                                                          28 PSI    ±1 PSI                                        15. Conveyor Speed 2.5 Ft/Min.                                                                             ±0.5 Ft/Min.                                      (Cleaner)                                                                 ______________________________________                                    

A wave length of about 16 inches best accommodates the pallet 50 whichhas a width of 9.85 inches and a length of 15.73 inches. The conveyorspeed has been lessened from about 4 ft/min. to about 2.5 ft/min. tolessen the chance of having shorts between leads. This also affordsoptimum temperature of the solder for proper and optimum solder coatingof the lead.

Process development was verified by conducting tests on integratedcircuit packages held in a test fixture and passed through a solder waveproduced in an ASTRA 16 wave soldering machine with the J-leads in a"leads down" orientation and a "leads up" orientation. 84-lead Quadpackages were utilized with data taken on up to 10 leads, i.e. everyother, of the 21 leads on each side of the square 84-lead Quad package,i.e. a total of 40 leads. Forty tests were run on one package at the endof the elongate frame of packages and on the middle package of theelongate frame of packages so as to be representative of solder coatingconditions across the wave which the elongate frame of packagessimultaneously traverse. For example, in test Lot No. 1-1-1 on thecenter package #3 with the leads facing downwardly on an ASTRA 16apparatus, the lead crest thicknesses, as measured by x-ray fluorescenttechniques, of measured leads varied from 591.9 microinches to over 1000microinches. At the 1000 microinch point an alarm indicated a grossdeviation from a 500 microinches nominal desired thickness for 17 out ofthe 40 leads measured. The mean thickness of leads actually measured(those under 1000 microinches) was 820.8 microinches. By x-rayfluorescent techniques, it was found that the Sn constituent of thedesired 60% Sn and 40% Pb solder varied from 56.6% to 100% Sn. It wasclear that the solder coating was not within specifications forthickness, i.e. a range of thickness about 200 to 950 microinches andfor composition, i.e. 58-70% Sn.

Lot No. 2-1-1 were processed in the ASTRA 16 machine but processed inthe "leads up" orientation as shown in FIGS. 6 and 9. The solder coatingon the middle package of the group of five packages in an elongate framein all cases was below 1000 microinches in thickness with a meanthickness for all 40 leads tested of 480.9, very close to the nominalmean of 500 microinches desired. The mean of Sn composition at the crestof all 40 leads was 63.0% Sn with a minimum of 58.5% Sn and a maximum of69.9% Sn, again within the specifications given above. All of the abovepercents are in weight percent.

The above description of the preferred embodiment of this invention isintended to be illustrative and not limiting. Other embodiments of thisinvention will be obvious to those skilled in the art in view of theabove disclosure.

I claim:
 1. A pallet for mounting a series of integrated circuitpackages and for passing said mounted series of integrated circuitpackages through a wave of molten solder, said palletcomprising:elongate support members; spaced support bars attachedtraversely to said elongate support members; means for mounting a seriesof integrated circuit packages linearly in an elongate frame; means formounting a series of said elongate frames parallel to and between saidspaced support bars; and a series of package support bars extendingtransversely of said elongate support members for supporting ones ofsaid series of integrated circuit packages contained in each of saidseries of elongate frames.
 2. The pallet of claim 1 including means foraffixing each of said elongate frames between adjacent spaced supportbars.
 3. The pallet of claim 2 in which said means for affixingcomprises pins extending from said spaced support bars and apertures insaid elongate frames.
 4. The pallet of claim 1 wherein said series ofpackage support bars include space sets of at least two support pins forsupporting a flat top surface of each integrated circuit package held ineach of said elongate frames.